Years of widespread application of arsenic-based pesticides have reportedly increased the background concentration of this toxic metalloid in agricultural soils. Rapid encroachment of suburban development on lands previously used for agricultural purposes in the last two decades has tremendously increased the potential for human contact with this Group-A carcinogen. The importance of considering soil ingestion from incidental hand-to-mouth activity by children has been repeatedly emphasized in recent studies assessing public health risks associated with long-term exposure to low-level metal-contaminated systems. The long-range objective of the proposed research is to help develop a more accurate risk assessment model for exposure to low doses of arsenic in soils. Studies suggest that bioavailability of arsenic is much less in soils than in water (100% bioavailable), indicating that the current practice of assessing human health risk from ingested soil-arsenic using the water model (due to absence of an appropriate soil model) seriously overestimates potential risk. It also sets much higher limits on soil-cleanup goals, essentially translating to millions of dollars in over-expenditure during the remediation process. In order to avoid overestimation of health risk, and to prescribe more appropriate and cost-effective remedial methods, an accurate assessment of bioavailability based on geochemical fate of arsenic in such soils is required. Realizing the heterogeneity of the soil-plant-water environment and the wide range of interactive bio-physico-chemical parameters, an integrated greenhouse and laboratory study has been proposed by a team of soil scientists, chemists, and plant scientists with the following specific aims: (1) to examine the relationship between geochemical speciation and bioavailability of arsenic as a function of soil properties, (2) to determine the applicability of quantitative models in predicting arsenic retention in complex multi-component systems, such as soils, (3) to evaluate the use of low-cost chemical amendments, such as water treatment residuals in decreasing soil arsenic availability, and (4) to identify the chemical, physiological and genetic mechanisms behind uptake and detoxification of arsenic in plant systems. Collectively, this new knowledge is expected to have a major positive impact on modification of the current human health risk assessment practices by understanding how soil biogeochemical properties influence arsenic uptake and bioavailability.